The paper manufacturing industry long has employed titanium dioxide pigment to enhance the optical properties, e.g., brightness and opacity, of various paper products. Traditionally, the titanium dioxide pigment which has been most widely employed, generally in high solids content slurry form, is dry milled anatase titanium dioxide. This pigment typically is produced by controlled hydrolysis of an aqueous solution of a titanium salt such as titanium sulfate and, after filtering and washing, calcined to dry the pigment and fully develop its anatase crystallographic form. For purposes of its use in paper manufacture, the calcined pigment is subjected to a further dry milling operation to break up oversized and agglomerated particles. The calcined and milled pigment then is dispersed in water in amounts sufficient to produce slurries having solids contents ranging from 60 to 80 percent by weight.
Rutile titanium dioxide pigments also are employed in paper manufacture and their use is becoming more prevalent as the production of anatase titanium dioxide pigments by the so-called "sulfate process" described above diminishes. Generally, the rutile pigments are prepared by oxidizing titanium tetrachloride in the vapor phase. The resulting oxidation product is a crude titanium dioxide pigment containing substantial amounts of oversized, gritty titanium dioxide particles. To render this crude product suitable for use as a pigment, it subsequently is wet milled, hydroclassified, dried and finally subjected to a dry milling operation to produce a smooth textured pigment product. When used in the manufacture of paper, this dried and milled pigment product is dispersed in water in amounts sufficient to provide slurries having solids contents in the range disclosed hereinabove for anatase pigments.
Whether the titanium dioxide pigments employed in preparing the high solids content slurries are prepared by the above generally described sulfate process or oxidation process, typically they are prepared from dry, finished pigments, i.e., pigments that have been subjected to drying and dry milling operations. One exception to the typical use of finished pigment is disclosed in U.S. Pat. No. 3,758,322. This patent describes a process for producing high solids content slurries using an in-process, low solids content slurry of crude titanium dioxide pigment which pigment has been wet ground and subjected to a grit removal step (i.e., classification) but no more. The process described comprises flocculating the grit-free, low solids content pigment slurry, dewatering said flocculated pigment slurry under vacuum and reslurrying the dewatered pigment to produce a slurry of at least 60% solids by weight. Although the process described is asserted to be more cost effective, as a result of the elimination of the costly and time consuming titanium dioxide finishing steps, it is not without its own disadvantages. One disadvantage is the added costs, in time and materials, associated with the need to flocculate the pigment to render it filterable utilizing conventional vacuum filtration techniques. Another disadvantage is the lower production rates associated with the process as the result of the long dewatering cycles required to produce the high solids content filter cakes used to prepare the final slurries.
The invention described hereinbelow overcomes the above disadvantages by eliminating the need for a flocculation step and the long dewatering cycles required to produce the high solids content filter cakes from which the final slurries are prepared.